1. Field of the Invention
The present invention relates to switching systems and more particularly to switching systems that require switching between multiple signal sources and multiple signal sinks. It is particularly useful for modular, multiple radio systems. It utilizes a loop segment switching scheme that obviates the requirement of matrix switching.
2. Description of the Related Art
Modern modular radio systems typically utilize a “pool” of exciters to generate low power signals with a selected frequency and modulation, and a “pool” of power amplifiers which increase the low level signal from an exciter to the desired radiated power level. The system generally maps one exciter to one power amplifier. Connecting more than one exciter to a single power amplifier raises issues with linearity versus generation of spurious intermodulation distortion (IMD) products, and peak envelope power (PEP) derating of the amplifier with multiple signals (whereby the vector voltage limited power amplifier must have the input signals reduced such that the peak vector voltage input of multiple signals times the amplifier gain does not exceed the peak voltage output capacity of the amplifier to prevent severe IMD product generation).
In designing modern modular radio systems the designer is faced with an architectural challenge based on the fact that the system configuration uses modular exciters and banded amplifiers. There is a choice as to whether the exciter is hardwired to the amplifier or whether to provide a switch matrix to allow connection of any exciter output to any amplifier input. The reason for wanting to switch them is for fault tolerance, and being able to reroute around failed modules.
One of the key features of the new modular radio systems (typified by the US Department of Defense Joint Tactical Radio System (JTRS) is that the exciter module is capable of operating at output frequencies of 2-2000 MHz with selectable software-generated bandwidths and modulations. That is to say that the exciter is common across all frequencies, modes and waveforms. Modes and frequencies may be changed in multiples of microseconds to milliseconds without physical action; all features are switchable/selectable via software command busses.
On the other hand, the high power RF and antenna subsystem is generally still banded due to component and physics limitations. Typically, a military radio system is divided by frequency bands and their unique band attributes as follows:
HF (2-30 MHz) has 1000 watts transmitter output or more due to link power budget and atmospheric noise considerations, with physically large (35-50 feet tall) antennas commensurate with the wavelength; narrowband linear 3 KHz waveforms.
VHF Combat Net Radio (30-88 MHz) has 50 watt transmitters with 8-10 foot tall antennas, operates in line-of-sight (LOS) modes to manpack or handheld radios from larger base stations on platforms or movable command posts; non-linear narrowband waveforms on 25 KHz channelization.
UHF (225400 MHz) has 100 watt transmitters with 2-3 foot tall antennas, operates in LOS modes among platforms and movable command posts; linear and non-linear narrowband waveforms on 25 KHz channelization except special waveforms up to several megahertz bandwidth.
UHF SATCOM (240-318 MHz) has 200 watt transmitters with steerable directional antennas; linear and non-linear narrowband waveforms of 5 or 25 KHz; four 200 watt transmitters combined into one antenna via −6 db combining network, net of 50 watts per channel radiated power.
High UHF (>400 MHz) uses 100 watt transmitters (with exception of JTIDS, with dedicated higher power transmitters and antennas) with 1-2 foot tall antennas; linear and non-linear waveforms up to ten's of megahertz wide.
The point of the description of the transmitters used is that each band has a unique power output and power amplifier requirement. With modern technology and these differing requirements, the amplifiers generally used to satisfy the total frequency coverage needs include:
HF: Due to the high power needs, the HF amplifiers are segmented as separate devices operating only 2-30 MHz. High power devices with sufficiently large junctions typically have unsatisfactory characteristics for VHF/UHF operation.
VHF/UHF: The state-of-the-art allows construction of 30450 MHz 100 watt amplifiers, both linear and non-linear, using LDMOS silicon devices. Two or more amplifers are coherently hybrid-combined for higher power outputs (i.e., for satellite communications). Current LDMOS silicon devices cannot be broadbanded to obtain 30-2000 MHz in a single amplifier.
>450 MHz UHF: The state-of-the-art is shaky in this frequency range. Silicon devices are usable up to 2000 MHz, but due to the rapidly decreasing gain (up to a usable limit of about 2000 MHz), must be tuned and cannot be broadbanded except over a small band (typically 5-10% of tuned frequency). GaN and GaAs devices are not yet available with sufficient power density, and SiC devices are just being introduced into the market. Hence this band is typically covered with banded amplifiers.
Thus, in summary multiple banded amplifiers are required to meet the operational requirements of present military radio systems. However, one common exciter module can generate low level signals for all bands.
A particular example of the problem facing large multiple radio systems involves those found on military ships that may have upwards of twenty-five to fifty radios. These are traditionally connected to an RF crosspoint matrix switch to allow the system to reroute around failures and maintain critical communications circuits in the face of those failures. Newer systems being proposed under the Joint Tactical Radio System (JTRS) program may have closer to one-hundred channel radio systems, using common modular components, requiring a 100×100 RF matrix switch to achieve the traditional reconfigurability. Such a large size RF matrix switch will be heavy, expensive, and itself a reliability risk relative to the systems it is to reroute.
In attempting to determine whether to hardwire an exciter to an amplifier, or provide a switch matrix to allow connecting any exciter output to any amplifier input some of the factors to be considered are:
What is the mean time between failure (MTBF) and availability of the system with hardwired versus switched exciter/amplifier pairs?
What is the cost of an exciter crosspoint switch matrix?
Is the baseband switch at the digital input of the exciter sufficient switching, given that all exciters can operate at any mode and frequency?
This becomes a complex system trade. The motivation for wanting to switch any exciter to any amplifier/antenna must be understood, given that any exciter can generate any waveform at any frequency. The primary motivation is that the failure of an exciter would leave an unused amplifier/antenna channel. This could be resolved by matrix-switching a replacement or spare exciter into that amplifier, or simply having sufficient exciter and amplifier spares in each band to provide the required reliability/availability without switching (e.g., hardwired).
Assuming for discussion that switching is justified, then large systems (64 to 80 exciters) which require connecting any exciter to any amplifier would require a commensurate crosspoint matrix, which is a large and expensive hardware item, albeit one available as COTS.
Although the discussion above has been directed toward modular, multiple radio systems similar switching problems are equally applicable in many areas where switching is required between multiple sources and multiple sinks such in optical switching, data communications, energy related systems, etc. As will be disclosed below, the principles of the present invention apply equally to a variety of fields.